Using JSON schemas which are not described by a JSONSchema meta schema

This is a feature request i guess, if I upload the following schema:

```json
}
"$schema": "http://localhost:8040/api/v1/schemas/CDCSchema",
"$id": "http://localhost:8040/api/v1/schemas/CUG",
"title": "CUG",
"type": "object",
"...": "..."
}
```
an internal error message is generated: `Error validating JSON schema: Unknown MetaSchema: https://localhost:8040/api/v1/schemas/CDCSchema`. (This message is not in the response (which is ok^^).)

It would be helpful if schemas can be used that are not directly described by a JSONSchema meta schema. For the beginning it would be enough (at least for me) to consider only meta schemas that are located in the store instance. =)